These investigations are aimed at better understanding the regulation of fatty acid biosynthesis in normal rat mammary gland and mammary tumors. To achieve this objective the following studies are in progress. a) Whereas the activity of hepatic acetyl CoA carboxylase has been shown to be regulated by phosphorylation/dephosphorylation of the enzyme molecules, it is not yet certain whether the activity of rat mammary gland acetyl CoAcarboxylase is also controlled by a similar mechanism. Therefore studies will be conducted on the regulation (short-term) of rat mammary gland carboxylase. b) The regulation of acetyl CoA carboxylase and fatty acid synthetase in explants of normal mammary gland and mammary tumors will be investigated. c) Using certain acyl CoA analogs, the mechanism of reaction catalyzed by rat mammary gland fatty acid synthetase is being explored.